[Laser] Polarization Subtraction

[Dave]

On 2012-10-18 12:02 PM, laser-request at mailman.qth.net wrote:
> Date: Wed, 17 Oct 2012 21:02:38 -0700 (PDT)
> From: Tim Toast <toasty256 at yahoo.com>
> Subject: Re: [Laser] Polarization Subtraction
> To: laser at mailman.qth.net
> Message-ID:
> 	<1350532958.66967.YahooMailClassic at web161003.mail.bf1.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Thanks for all the responce to this,
>
> The two lens idea was mine and i tossed it in there with all
> that. The article though mentions a single lens with a special
> beam splitter. Yes, i think now the slight difference in angle
> of a two lens capture would lead to a sort of non-simultaniousness
> that is not intended by the article. Not to mention a totally different, although similar, scene instead of "the same scene".
> It sounds to me that they are using a single lens with a beam
> splitter to get the two polarised images. Either a polarizing
> prism type splitter that provides (I think) one polarized image
> and one unpolarized one which would need a separate filter of its
> own to provide the two orthogonal images -or- a plane beam
> splitter that each image would need a polarizing filter.
>
> The key element is "at the same time" and not sequential though -
> which negates my thought that two lenses might work or some sort
> of alternate frame capture for a ccd.

I had envisioned two CCDs driven from the same clock sources, such
that their images would be read out simultaneously.  But, the CCD approach
seems overly complex if all of the information could be supplied by two
photocells.

> You would think though that
> any polarization changes going on would be slow ones at rates less
> than say, 100 Hz but i don't know for sure things aren't
> happening much quicker than that.

If they're using a mechanical modulator, then the polarization change is
probably reasonably slow.  But, if they're using something like a
Pockels cell for the change of polarization modulation, then it may be
VERY fast (few nS!):

http://en.wikipedia.org/wiki/Pockels_cell#Applications_of_Pockels_cells

Of course, there are other modulators:

http://en.wikipedia.org/wiki/Kerr_effect
http://en.wikipedia.org/wiki/Rapatronic_camera

http://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect

http://en.wikipedia.org/wiki/Faraday_effect

> Things aren't always as they
> seem of course! We've learned that over and over :)
>
> -toast

Dave
WA4QAL